METALLURGICAL STRUCTURES OF AS-CAST AND HEAT-TREATED HIGH-PALLADIUM DENTAL ALLOYS

Scanning electron microscope observations and energy-dispersive spectr oscopic analyses have been performed on two first-generation and two s econd-generation high-palladium dental casting alloys. A specimen desi gn simulating a maxillary central incisor coping was employed to conse rve metal, while providing thin and thick sections to yield a range of solidification rates. The alloys were centrifugally cast in air, foll owing standard dental laboratory techniques; three castings were prepa red for each alloy. Each casting was sectioned to produce two mirror-i mage specimens, and one specimen received the appropriate oxidation he at treatment, followed by a simulated full porcelain firing sequence. After metallographic polishing, specimens were examined with a scannin g electron microscope. The as-cast alloys displayed multi-phase micros tructures which could be explained by the rapid solidification conditi ons and the relevant phase diagrams. The simulated porcelain firing he at treatment caused a variety of bulk microstructural changes in the c oping sections, along with formation of complex subsurface oxidation r egions which were less thick for the second-generation alloys. Element al compositions of the palladium solid solution matrix in the heat-tre ated alloys were in good agreement with nominal alloy compositions pro vided by the manufacturers. Ruthenium-rich particles found in the micr ostructures of three alloys are consistent with a proposed mechanism f or grain refinement.